Generally, fuel injection of vehicle engines are categorized into PFI and GDI. The PFI is an injection method which is mainly used in gasoline engines, and is an injection method that injects low pressure fuel into an intake port to supply a mixed air including air to the inside of a cylinder. The GDI is an injection method which is mainly used in diesel engines, and is an injection method that directly injects high pressure fuel into a cylinder. Hereinafter, an engine using the PFI is referred to as a PFI engine, and an engine using the GDI is referred to as a GDI engine.
In a partial load, the GDI engine injects fuel at a last stage of a compression stroke and thus easily ignites even at an ultra-lean air-fuel ratio through stratified charge combustion which allows an air-fuel ratio around an ignition plug to be sufficient. In a high load, the GDI engine injects fuel at an initial stage of an intake stroke and thus cools intake air through an air-fuel ratio for complete combustion, thereby enhancing filling efficiency. The GDI engine directly injects fuel into a cylinder, and thus, decreases a wall wetting phenomenon where fuel is adsorbed onto an intake port wall.
Despite such advantages, the GDI engine injects fuel into a cylinder in an intake stroke section, and thus, the GDI engine is lower in homogenization performance than a conventional PFI engine. Accordingly, in gasoline engines, a complex injection engine where the PFI and the GDI are combined has been developed.
As described above, since the PFI engine uses a method of injecting low pressure fuel into an intake port, in a fuel supply system based on the PFI engine, low pressure fuel to which fuel stored in a fuel tank is changed is transported to a low pressure injector that injects the low pressure fuel into the intake port, and thus, a low pressure fuel supply line that transports the low pressure fuel to the low pressure injector is required to be developed. Additionally, since the GDI engine uses a method of injecting high pressure fuel into a cylinder, high pressure fuel to which fuel stored in a fuel tank is changed is transported to a high pressure injector that injects the high pressure fuel into the cylinder, and thus, a high pressure fuel supply line that transports the high pressure fuel to the cylinder is required to be developed. Therefore, in a related art fuel supply system for complex injection engines in which the PH and the GDI are combined, the low pressure fuel supply line and the high pressure fuel supply line should be simultaneously designed, and for this reason, designs of all fuel supply lines is complex.